Replace assertions involving calls to isset() on xpath results in functional tests with WebAssert calls

Problem/Motivation

Split from #3131807: [meta] Replace assertions involving calls to isset() with more appropriate assertions

Proposed resolution

-    $relations_widget = $this->xpath("//details[@id='edit-relations']");
-    $this->assertTrue(isset($relations_widget[0]), 'Relations widget element found.');
+    $this->assertSession()->elementExists('xpath', "//details[@id='edit-relations']");

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

None